In some countries, such as Scandinavian countries, it is a legal requirement for the parking brake of a vehicle to be releasable when filling compressed air only if the vehicle driver performs this action manually, that is, by operating a lever, valve, or switch, for example. This is intended to prevent the vehicle from rolling away without the vehicle driver. Although preventing the vehicle from rolling away is not necessarily a legal requirement, vehicle drivers, in particular in Scandinavia, have become accustomed to this safety function, and therefore vehicle manufacturers continue to provide this functionality.
A dangerous situation of this kind can occur, for example, in the case of a utility vehicle that is braked using compressed air, if the vehicle has been parked on an incline and the compressed-air storage means have been depressurized overnight. Although the vehicle is held in its existing position by the parking brake, which is designed as a spring-loaded brake, by virtue of spring force, as soon as the vehicle driver starts the engine of the vehicle, the compressed-air storage means are filled in a defined order. In the process, the compressed-air storage means of the service brake circuits are usually first filled with compressed air in order to ensure that the vehicle can be braked at any time. The spring-loaded brake would only then be supplied with compressed air, resulting in the release of the spring-loaded brake. Therefore, if the vehicle driver were to start the engine and wished to release the parking brake by means of an operating device in the cab of the vehicle, the vehicle would not begin to move since the compressed-air storage means of the service brake circuit first have to be filled. If the vehicle driver then leaves the cab in order, for example, to find the fault preventing the vehicle from moving away or, for example, to clean the windshield, the compressor that supplies compressed air to the compressed-air storage means would continue to deliver compressed air during this time. As soon as the compressed-air storage means of the service brake circuits are filled, the parking brake would then also be supplied with compressed air, which would release the parking brake and allow the vehicle to move. There is a danger here of the vehicle driver and/or other people being run over by the driverless vehicle.
In order to prevent this, various mechanical solutions employing further pneumatic valves have been provided to date. One disadvantage of a mechanical solution of this kind is that different systems have to be provided for different countries, resulting in higher costs being incurred.